Why Is My Kombucha Not Fizzy? Fix Flat, Uncarbonated Kombucha

Flat kombucha after second fermentation is one of the most common frustrations in home brewing. The good news is that it almost always comes down to one of a small number of fixable causes. Work through them from most to least common and you’ll usually find the culprit fast.

Why carbonation happens (and why it doesn't)

Second fermentation carbonates kombucha by creating a sealed environment where the CO₂ that yeast produces can’t escape. The gas dissolves into the liquid under pressure — that’s carbonation — and comes back out as bubbles when you open the bottle. For it to work, four things all have to be true at once:

Enough residual yeast the culture suspended in the bottle has to be alive and active.

Sugar to consume the yeast needs fermentable sugar to produce CO₂.

Bottles that hold pressure no leaks — CO₂ must be trapped, not escaping.

Warm enough temperature yeast has to be active, which means a warm room.

If any one fails, you get flat kombucha.

The most common causes, in order

1
Bottles that do not hold pressure The single most common cause. Worn swing-top gaskets, loose screw-tops, or chipped rims leak CO₂ as fast as the yeast makes it. After sealing, press the cap and listen for hissing, or briefly submerge the sealed bottle upside down and watch for escaping bubbles. Replace gaskets and retire damaged bottles.
2
Not enough sugar for 2F If your 1F went very long or very warm, most sugar is already gone. Add 5–10g white sugar per 500ml (1–2 tsp) or 30–50ml fruit juice per bottle at bottling. The tell-tale sign is a 1F that tasted sharply vinegary before bottling.
3
Room temperature too cold Below 18°C (65°F), yeast goes sluggish and fizz builds slowly or not at all. Move bottles to 22–26°C — top of the fridge, a cupboard, or near (not on) a heat source — reseal, and give it 2–3 extra days.
4
Second fermentation time too short Opening after 24 hours is rarely enough. Most 2F needs 2–5 days. Leave bottles at least 3 full days, tasting and burping carefully once a day.
5
Very acidic 1F stalling the yeast Less common: a very low-pH 1F can suppress the acid-tolerant-but-not-acid-proof yeast enough to slow carbonation. Pull your next 1F a day or two earlier, tasting daily from day 7.

What to do with flat bottles right now

If bottles have sat 5+ days and still feel soft, you can usually rescue them.

1
Add a little sugar Open each bottle and stir in about 1 tsp of sugar per 500ml, then reseal.
2
Check the seals Replace worn gaskets and make sure caps tighten fully — a leak here is the usual reason nothing built up.
3
Move somewhere warmer Relocate to a 22–26°C spot away from cold corners and exterior walls.
4
Wait and burp Give it another 3–4 days, burping once daily. If after two attempts they still will not carbonate, the yeast is likely depleted — drink it still; it still tastes fine.

Preventing flat kombucha next batch

A few habits make good carbonation the default rather than a gamble: pull your 1F when it tastes pleasantly tart rather than vinegary so more residual yeast activity remains; add a measured amount of sugar or juice at bottling instead of eyeballing it; use bottles with intact seals; keep 2F bottles in a warm room, not a cool corner; and wait at least 3 days before checking.

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Second Fermentation Calculator

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Frequently asked questions

How long should second fermentation take?+

At room temperature (20–24°C / 68–75°F), second fermentation typically takes 2–5 days. Warmer rooms carbonate faster — sometimes in 24–48 hours. Cooler rooms can take 5–7 days or more.

Can I carbonate kombucha in plastic bottles?+

Yes. Plastic bottles work well and have a practical advantage: you can feel the pressure building as you squeeze the bottle. When it's firm and resists squeezing, carbonation is usually good. Transfer to glass for storage and serving if preferred.

My kombucha is fizzy in the bottle but flat in the glass — why?+

This usually means the kombucha is over-carbonated and releasing CO₂ rapidly when poured. Pour slowly into a chilled glass, keep the bottle cold before opening, and don't shake. Very tart kombucha can also lose fizz quickly once poured.

Does first fermentation produce any carbonation?+

Yes, a small amount. First fermentation kombucha is usually lightly effervescent, but the breathable cover allows CO₂ to escape so pressure doesn't build. Significant carbonation requires sealed bottles in second fermentation.

How can I tell a bottle is ready without opening it?+

With plastic bottles, squeeze — firm and resistant means good pressure. With swing-tops, the bottle feels taut and gives a solid hiss when you crack the seal. When in doubt, chill one test bottle and open it to judge the fizz before refrigerating the rest.